1. Field of the Invention
The present invention relates to an aqueous ink composition for writing instrument, and more particularly to aqueous ink adapted for use in a ball point pen.
2. Related Background Art
Conventional ball point pens are generally classified into oil ball point pens utilizing highly viscous oily ink and aqueous ball point pens utilizing aqueous ink of low viscosity, but highly viscous aqueous ink is recently proposed for the aqueous ball point pens.
Such highly viscous aqeuous ink maintains a high viscosity under a weak shearing force, but exhibits a low viscosity under a high shearing force, as generated by the rotation of the ball in the writing operation of the ball point pen.
As examples of the above-mentioned proposal, the Japanese Patent Publication No. 64-8673 discloses the use of xanthane gum, and the Japanese Patent Laid-open Application No. 4-214782 discloses the use of wellan gum, for increasing the viscosity of aqueous ink.
However, it is difficult, in such gum-containing aqueous ink, to maintain long-term stability of dispersion of the pigment constituting the coloring material, and the ball point pen employing such ink may result, in the writing after a prolonged time, in a pale or excessively dense record, and may eventually become incapable of writing operation due to the ink blocking at the pen tip by pigment coagulation.